This invention relates to diffraction by the use of periodic structures such as crystals, gratings and the like and more particularly to an instrument for diffraction in which the spacing associated with the crystalline planar or grating elements in the periodic structure is progressively increased or decreased along a face or direction of the structure to progressively change the Bragg diffraction angle, and to a method of providing a controlled and progressive change in the Bragg angle along a face or direction of the structure associated with diffraction spacing to increase the usable diffraction area or acceptance angle of the structure for monochromatic radiation and thereby improve the extent that beams of photons and particles may be focused or otherwise controlled.
The diffraction of photons such as x-rays and gamma rays by crystals is an old and well established discipline. Crystal diffraction may generally be divided into two classes, the "transmission" type and the "surface diffraction or reflection" type. In the transmission type as illustrated in the schematic diagram of FIG. 1a, the crystal planes used in the diffraction process are perpendicular to the face or incident surface of the crystal and the beam of photons pass through crystal. In the "surface diffraction or reflection" type as illustrated in the schematic diagram of FIG. 1b, the crystal diffraction planes are parallel to the face or incident surface of the crystal and the beam of photons are diffracted near this surface so that they merge from the same face of crystal that they entered. The transmission type diffraction is used mainly for high energy photons with their corresponding small Bragg angles while the surface type diffraction is more useful with lower energy photons with their larger Bragg angles and higher absorption coefficients.
Early diffraction instruments such as the spectrometer used flat crystals and had efficiencies as low as 10.sup.-9 diffracted photons per source photon. The low efficiency occurred because only a very thin slice of the crystal satisfied the Bragg condition for the diffraction based on the Bragg equation EQU n.lambda.=2d sin .theta.
where "n" is the order of diffraction, ".lambda." is the wavelength of the photons, "d" is the crystalline plane spacing, and ".theta." is the Bragg angle or incident angle. If the beam of photons entered the crystal at an angle other than the Bragg angle, reflection at that portion of the beam was essentially eliminated. For purposes of illustration, the usable narrow slice of a crystal may be only about 0.001 cm for a high quality crystal with a rocking curve of about 2 seconds and with a source at a distance of about 100 cm.
Some of the important features of crystal diffraction instruments relate to the extent that the beam of photons (i.e., x-rays and gamma rays) or particles (i.e., neutrons) may be diffracted with a reasonable efficiency and focused or otherwise controlled to provide an image of desired intensity. Since the usable area or acceptance angle of flat crystals is extremely limited, it has become necessary to bend crystals to improve the area or acceptance angle over which the Bragg condition was satisfied to improve the efficiency and intensity levels of the diffracted beam. The schematic diagrams of FIGS. 2a and 2b provide illustrations of bent crystals used for the transmission and reflection type of crystal diffraction. While the use of bent crystals improved efficiencies, intensities, and focusing operations of the crystal diffraction instruments over those for instruments using flat crystals, it was not always possible to easily bend crystals to the desired extent and some crystals such as those of bismuch and tin would tend to break before being bent beyond a limited extent.
Further, the crystal diffraction instruments with bent crystals had disadvantages. As illustrated in the schematic diagram of FIG. 2a with the transmission type, it was necessary to use a broad source to provide a concentration of monochromatic radiation at a line image. With the reflection type, as illustrated in the schematic diagram of Fig 2b, it was possible to form a focused line image from a point source although the distances of the image and source usually were equidistant from a center line.
Focusing is of considerable importance to instruments using crystal diffraction since accurate detection and measurement of diffracted beams often are dependent on the intensity of the diffracted beam and the extent that the beam is focused within a small area. As illustrated in FIGS. 1a and 1b for beams which are not effectively focused, the target or image area must be increased for effective detection or measurement.
Focusing of parallel rays is also of importance. In the telescope in the Einstein satellite which has been in orbit around the earth, total reflecting mirrors are used to focus parallel beams of x-rays and gamma rays from deep space. Limitations in the performance of the reflecting mirror system limited the usable photon energies for this satellite telescope to about 5 KeV and below with more satisfactory performance being at about 2-3 KeV. Increase in the usable photon energies to values above about 5 KeV would be desirable. Replacement of the mirror system with crystal diffraction systems in the present state of the art would not solve these problems since they do not effectively focus parallel rays, even those of low photon energies. Therefore, new crystal diffraction systems with improved performance in focusing or converging parallel rays at higher photon energies would be desirable for satellite telescopes and other instruments.
In a similar manner, diffraction gratings have become important for the focusing and imaging of soft x-rays, ultraviolet, visible and infrared radiation. The basic difference in these methods for focusing is that diffraction occurs in the grating by a two dimensional phenomena while it is three dimensional in the crystalline structure. Diffraction gratings are conventionally made by photographic techniques to produce a series of parallel lines in the film and by etching or machining of conductive metals to produce a similar pattern in the metal surfaces.
Since gratings have conventionally been made with the diffraction spacing being essentially constant, the effectiveness of these gratings has been limited for much the same reasons that were discussed above for the crystal diffraction case. The constant diffraction element spacing results in a constant diffraction angle for the diffracted beam. This makes it impossible to convert a parallel beam into a convergent beam and/or to use the diffraction process as a method for focusing radiation from any type of source except in the very special case of the reflection type diffraction grating used in the zero order (.theta..sub.1 =.theta..sub.2) where no spectral discrimination occurs.
One of the objects of this invention is to provide a means of increasing the area or acceptance angle in periodic structures used for crystal diffraction and in grating diffraction. Another object is to increase the efficiency of the diffraction process. An additional object is to improve the intensity of the diffraction process. A further object is to improve focusing in instruments utilizing crystal diffraction or diffraction by gratings. Yet another object is to provide means for focusing of parallel beams. It is also an object to increase the energy levels to values above 5 KeV for focused beams which may be diffracted by diffraction instruments. These and other objects will become apparent from the following description.